Headers for 1966 Barracuda 273 Commando formula S

Yup.
Doug's D450/D450-B
Sanderson DD9
Layson's carries the Spitfires.
You might be able to find some old used Hooker or Headman fenderwell headers if you don't mind cutting out your inner fenderwells.
Blockhugger or Shorty headers don't fit without some rather drastic modifications:
Removing stock exhaust manifolds